The present invention relates to an image sensing apparatus including an image sensor for generating an electrical signal corresponding to an amount of incident light and particularly to an image sensing apparatus and an image sensing system using an image sensor whose photoelectric conversion characteristic has a first area in which the electrical signal is outputted by being converted according to a first characteristic in relation to the amount of incident light and a second area in which the electrical signal is outputted by being converted according to a second characteristic different from the first characteristic in relation to the amount of incident light (e.g., operation by a linear characteristic and operation by a logarithmic characteristic can be switched), and an image sensing system operating program product.
There have been known image sensors (also called “LOG sensors”) constructed by adding a logarithmically converting circuit including a MOSFET or the like to a solid-state image sensing device in which photoelectric conversion elements such as photodiodes are arrayed in matrix and adapted to set such an output characteristic of the solid-state image sensing device as to logarithmically convert the electrical signal in relation to an amount of incident light using a subthreshold characteristic of the MOSFET. Among such image sensors is known the one in which an original operative state of the solid-state image sensing device, i.e., a linear operative state where an electrical signal is outputted by being linearly converted in relation to an amount of incident light and the aforementioned logarithmic operative state can be switched to each other.
For example, Japanese Unexamined Patent Publication No. 2002-77733 discloses an image sensing apparatus constructed such that a specific reset voltage is supplied to a MOSFET to enable the automatic switch from a linear operative state to a logarithmic characteristic area and to set the equal switching points for the respective pixels. Japanese Unexamined Patent Publication No. 2002-300476 discloses an image sensing apparatus constructed such that an operative state can be automatically switched from a linear operative state to a logarithmic characteristic state and a potential state of a MOSFET is made adjustable by adjusting a resetting period of the MOSFET.
In the case of using the image sensor in the linear characteristic state, an output proportional to an amount of electric charges produced by the photoelectric conversion elements is obtained. Thus, there is an advantage that a high-contrast image signal can be obtained for a subject having a low luminance, whereas there is a disadvantage that a dynamic range is narrower. On the other hand, in the case of using the image sensor in the logarithmic characteristic state, an output natural logarithmically converted in relation to the amount of incident amount is obtained. Thus, there is an advantage that a wide dynamic range can be ensured, whereas there is a disadvantage that contract is poor since the image signal is logarithmically compressed.
Generally, in a digital image sensing apparatus, various digital signal processings including a white balance correction, a color interpolation and a gradation conversion are applied to an image signal obtained by an image sensor. Here, the gradation conversion is such that gradation characteristics (γ-curve and digital gain) of image signals of color components R, G, B are so corrected as to match the gradation characteristics of an LCD device provided in the digital image sensing apparatus, a monitor television to which the image signals are externally outputted or the like.
However, in the case of using the image sensor provided with the linear characteristic area and the logarithmic characteristic area as above, there is a problem that no gradation conversion true to a subject luminance can be carried out if the gradation is uniformly converted over the entire sensing enabling area of the image sensor. More specifically, since the electrical signal is outputted by being natural logarithmically converted in relation to the amount of incident light on the image sensor in the logarithmic characteristic area, the image is sensed while having its gradation compressed in accordance with the incident luminance, resulting in the image sensing with a gradation characteristic different from the one in the linear characteristic area. Particularly, in view of the advantages and disadvantages of the linear characteristic area and logarithmic characteristic area, it is desirable to construct the image sensor such that an image sensing operation is carried out actively making the most of the advantages of the respective operative states. For example, even in the case of carrying out an automatic exposure control, there is a possibility of carrying out an optimal automatic exposure control making the most of the advantages of the respective operative states if this control is carried out while relating the subject luminance and a switching point of the image sensor from the linear characteristic state to the logarithmic characteristic state as targets. However, in such a case, since a control unavoidably constantly changes the switching point (inflection point) between the linear characteristic area and the logarithmic characteristic area, the optimal gradation conversion in accordance with the subject luminance becomes even more difficult. The above two publications merely disclose the image sensing apparatuses capable of automatically switching the operative state of the image sensor from the linear characteristic area to the logarithmic characteristic area, but do not mention about such a gradation conversion.